I got a vehicle mod working and fixed the error thats common with them. I heard someone say its because of the fact that the modded vehicles are larger sizes and the game thinks it cant load that many in the memory. However. Im getting gnar lag and missing textures and such. Does anyone know of a better performance mod or perphaps a way to tune the graphics on GTA IV?

You could try RealizmIV. It supposedly has better textures at better framerates. CLICK HERE to get it. Unless you are running an ENB setup, in which case you should not use a full install of Realizm IV, although you can use some of the textures if you wish.

Other than that, lower the view distance, detail distance, vehicle density and shadow density settings (especially view distance and shadow density, these two have a huge impact on performance). If you are running an ENB setup, you might try running with Ambient Occlusion Off, just edit the enbseries.ini/iceconfig.ini and switch EnableAmbientOcclusion to false. If the ENB setup you are using came with Anti Aliasing turned on and you are running at a high resolution, try turning this down or off altogether. The settings in enbseries.ini/iceconfig.ini for this are ForceAntiAliasing (set to false to turn off) and AntiAliasingUpscale (set it to 0.87 for a performance boost if anti aliasing is on).

If you still can't get performance you are happy with, maybe you should uninstall a few mods. Unfortunately, all these cool mods, especially enb mods, increase the performance requirements of GTA IV somewhat. Nearly all mods give you higher polly models, better pixel shaders etc. and this is always going to cost you in terms of performance.

when you start to get taxi bugs, missing textures, ped bugs, or any time of texture or actor/object loading bug it's because the game is running out of memory and is trying to compensate for the loss of memory by the mods. the rage engine is neat in that since the engine doesn't understand where the extra memory weight is coming from (i.e. car packs and enbs) it compensates and tries to continue on and make the game playable by shortening the amount of actors and objects it needs to load and process. the best remedy is to buy a graphics card that has 2GB or 3GB of dedicated memory so you can use up 1GB for all the bells and whistles of the game in it's vanilla form, and then use the rest of the memory for car packs and enb effects. you might have memory left over to, you know, run background stuff on your desktop.